Effects of intraspecific competition on the larval development and pupal weight of Dacini (Diptera : Tephritidae) infesting cucurbits in la Reunion

Background. In La Réunion, cucurbit crops suffer considerable damage due to fruit fly attacks. A complex of three species (The Melon fly, Bactrocera cucurbitae, The Ethiopian fly, Dacus ciliatus, and the Indian Ocean fruit fly, Dacus demmerezi) coexist in the island and can infest 16 different species of Cucurbitaceae. In order to develop sound and sustainable management methods for these pests, we need to know more about their behavior and ecology. The objective of this study is to clarify the interaction existing at larval stage, and particularly the competition which may occur between conspecific larvae in a same fruit. According to earlier studies, squash (Cucurbita pepo) was chosen for laboratory experiments because it is one of the most damaged cultivated cucurbit in La Réunion. Methods. To measure the effect of intraspecific competition on the survivorship rate of larvae and on the pupal mass, infestations of squash were carried out in the laboratory with L1 larvae of B. cucurbitae, D. ciliatus or D. demmerezi, obtained from lab rearing. Five levels of infestation were selected e.g. A) One larva (L1) for two grams of squash, B) One larva/1 g, C) Two larvae/1 g, D) Four larvae/1 g and E) Eight larvae/1 g. Each level was replicated five times. Four days after infestation, pupae were collected daily, weighed individually and grouped by weight classes in a same box and when they emerged, the adults were sexed. Results. Larvae of B. cucurbitae and D. demmerezi don't have the same response to an increase of intraspecific competition in a same fruit. Larvae of the first species show a good survivorship level (63% for level D) whereas larvae of D. demmerezi are more affected by a high level of competition (18% for level C). On the contrary, pupal weight of B. cucurbitae clearly decreases when the number of larvae increases in the fruit while pupal weight of D. demmerezi, does not seem to be affected by intraspecific competition. Conclusion. It seems that, in the complex of Dacini attacking cucurbits, the species do not have the same strategy regarding intraspecific competition. The larvae of B. cucurbitae are able to share resources and maintain a good survivorship finally giving more adults with low weight. Conversely, in D. demmerezi larval survivorship is decreased but the pupal weight of survivors remains high. Further studies will be needed on larval interspecific competition in this complex of fruit flies, in order to understand the performance of each species in situations of co-infestation. (Texte intégral

Performance of augmentorium as a sanitation technique against fruit flies (Diptera: tephritidae) in Reunion Island

Background. Tephritid fruit flies cause severe damage to fruit and vegetable crops in Reunion Island. Instead of the curative approach to reduce existing populations, the first step proposed for their management is sanitation. This method is based on an original technique firstly developed by USDA in Hawaii utilizing a tent-like structure called an "augmentorium" which aims to sequester adult flies emerging from infested fruit while allowing the parasitoids to escape, via a net placed at the top of the structure. This study focused on the performance and the efficiency of the augmentorium prototype recently tested in Reunion Island and particularly (i) the number of adult flies that can potentially be sequestered in an augmentorium in the field; (ii) the efficiency of the net mesh for fly sequestration and parasitoid escape; (iii) the feasibility of producing compost with infested fruit collected in the field. Method. The potential number of flies that could be sequestered was estimated by measuring in the lab the emergence of several species of flies from infested fruit collected in the field from 2005 to 2009 in different sites of the island. Emergence of adult flies was measured for six species of flies: (i) Bactrocera cucurbitae, Dacus ciliatus and D. demmerezi attacking three species of Cucurbits (pumpkin: Cucurbita maxima; cucumber: Cucumis sativus and courgette: Cucurbita pepo); (ii) Bactrocera zonata, Ceratitis rosa and C. capitata attacking one species of fruit (mango: Mangifera indica). The sequestration of three of these fly species (B. cucurbitae, B. zonata and C. capitata) and the escape of two of their parasitoids (Psyttalia fletcheri and Fopius arisanus) were assessed in the Cirad laboratory in Saint-Pierre in 2008. Preliminary tests on the feasibility of producing compost were then conducted in Saint-Pierre in 2009, mixing courgette and other components. Results. Collections of infested fruits showed the following means of emerged adults per kg of fruit: 76 for mango (B. zonata, C. rosa, C. capitata); 217 for cucumber, 340 for pumpkin and 594 for courgette (B. cucurbitae, D. ciliatus, D. demmerezi). The efficiency of the mesh chosen for the prototype of augmentorium (hole area 1.96 mm²) proved to be perfectly effective in the lab with 100% of sequestration of adult flies. In the same way, 100% of the parasitoids were able to escape from the mesh if they choose to do so. In addition, we showed that a ratio of 50:30:20 of courgette, sugar cane stem and chicken litter respectively was well adapted to produce compost. Conclusion. These results confirm the relevance and the efficiency of the augmentorium in an agroecological crop protection. As a sanitation technique against fruit flies, the augmentorium sequesters on average several hundreds of adult flies per kg of infested fruit. As a biological control method, it may contribute to increase parasitoid populations which are often low because of the previous and significant pesticide pressure. The augmentorium can also be considered as a useful tool to produce compost in the context of sustainable agriculture. The technique of sanitation using the augmentorium is now well accepted by farmers in pilot areas in Reunion Island. (Texte intégral

Spatial scales of genetic structuring in Bactrocera cucurbitae (Diptera, Tephritidae) : population structure on la Réunion

The genetic structure of B. cucurbitae was characterized at different spatial scales by considering (a) 25 worldwide-distributed populations (n=570) genotyped at 13 microsatellite loci and several samples genotyped at 2 mitochondrial gene fragments (COI, ND6), (b) 17 populations from East, West and Central Africa genotyped at 19 microsatellite and 3 mitochondrial gene fragments (COI, COII, ND6), (c) 2258 specimens sampled in La Réunion from different wild and cultivated host plants and at three altitudinal ranges and genotyped at 10 microsatellite loci and at 2 mitochondrial gene fragments. Microsatellite show that B. cucurbitae can be subdivided into five main worldwide distributed groups corresponding to populations from (1) the African continent, (2) La Réunion, (3) Central Asia, 4) East Asia and (5) Hawaii. The analysis of inter-regional assignments and the higher values of genetic diversity in populations from Pakistan, India and Bangladesh suggest that B. cucurbitae originated in Central Asia and expanded its range to East Asia and Hawaii on one hand and to Africa and the islands of the Indian Ocean on the other. Levels of genetic structuring within the African continent are much less pronounced. Still we can observe four main population groups colonizing Congo, West Africa, Sudan and East Africa, respectively. The Bayesian assignment of STRUCTURE shows further genetic structuring in East Africa, with populations from Uganda diverging from those of Tanzania and populations from Burundi and Kenya sharing admixture proportions with West African samples. In La Réunion, three distinct genetic clusters of B. cucurbitae are present which are different from the African and Asian groups. Genetically, these 3 clusters are poorly diversified and no recent bottleneck was observed. Results also suggest that these clusters show distinct spatial distributions in the island and that their genetic structure is linked with environmental factors such as altitude or rainfall but not with the nature of host plant (cultivated or wild). Extensive gene flow was inferred between clusters in La Réunion. Africa is the most probable source of the populations present in La Réunion, though the species in general is confirmed to be of Asian origin.The genetic structure emerging from the analysis of mitochondrial gene fragments is much less resolved with only two main groups of B. cucurbitae: one from Asia and Hawaii and the other from Africa and La Réunion. Overall, these data show increasing gene flow at smaller spatial scales and do not provide evidence of appreciable cryptic speciation within B. cucurbitae. (résumé d'auteur

Tritrophic interaction in the complexes of fruit flies damaging fruit and vegetable crops in Reunion island

The study of tri-trophic interactions is a vast field of basic ecological studies which are also of paramount importance for the development of environment-friendly methods of pest management. Within the Diptera, the Tephritidae family represents an interesting model for such studies because of the variable host-specificity of species in this group, and the important role that some natural enemies may play in the regulation of their populations. Furthermore, the great economic importance of many species in this family offers a large array of applications to research results. La Réunion, a French island situated in the south-west of the Indian Ocean, is a favourable area for studying such interactions, because of the number of tephritid species present in the island, and the great variability of its climatic conditions. On fruit crops, a complex of three polyphagous species of tephritids cause considerable damage: the Mediterranean fruit fly, Ceratitis capitata, the Natal fruit fly, Ceratitis rosa, and the Peach fruit fly, Bactrocera zonata. In addition, another complex of Dacini is harmful to cucurbit crops: the Melon fly, Bactrocera cucurbitae, the Ethiopian cucurbit fly, Dacus ciliatus, and the Indian Ocean cucurbit fly, Dacus demmerezi. Conversely, on Solanaceous crops such as tomato, the Tomato fruit fly Neoceratitis cyanescens, is the only species of economic importance. Over the last twenty years, many field studies allowed us to specify the host-range and relative importance of the different species on cultivated crops. Of particular interest are some host preferences observed in the field in some otherwise very polyphagous species, such as C. capitata. Detailed studies (lab, wind tunnel and field cages) were also devoted to the host location behaviour of stenophagous species, taking as a model the tomato fruit fly, N. cyanescens. Other behavioural studies also highlighted the preferences of the different species for particular host-plants during foraging or egg-laying behaviour. More recently, studies were focused on the quality of various host or non-host fruits for the pre-imaginal development of the different species, and its influence on their fitness. Though some indigenous parasitoids of the Mediterranean fruit fly have been recorded in the island, most of the natural regulation by parasitoids is due to exotic species imported through classical biological control programmes. This is particularly the case with two species imported from Hawaii, in collaboration with USDA Hawaii and the University of Hawaii: Psyttalia fletcheri, a larvo-pupal parasitoid of the Melon fly, and, more recently, Fopius arisanus, an egg-pupal parasitoid of some Bactrocera spp. Following the acclimatization of these two species, field studies allowed us to evaluate their host range, favourite habitats, and impact on host species populations. In addition, laboratory, field-cage and wind tunnel studies improved our knowledge of the stimuli involved in host habitat and host selection behaviours. Results of these studies are summarized and discussed in relation to the current state of knowledge of insect-plant and host-parasitoid interactions in tephritids, and to their possible applications in pest management. (Texte intégral

Caractérisation d'une résistance à la transmission du Tomato yellow leaf curl (TYLCV) par Bemisia tabaci dans les deux accessions sauvages apparentés à la tomate

Cette étude s'intègre dans ce projet de recherche et a pour but : i/ d'évaluer la résistance potentielle à la transmission du TYLCV présente dans les espèces sauvages Solanum pennellii LA716 et Solanum habrochaites LA1777 grâce à des inoculations de plants de tomates par des insectes virulifères et par voie mécanique (greffage). La comparaison des résultats obtenus permettra de séparer la composante " résistance à la transmission " de la composante " résistance au virus " sensu stricto. L'effet de doses croissantes d'insectes vecteurs sera également testé de façon à démontrer le caractère quantitatif de cette résistance à la transmission (contournement possible de la résistance à la transmission par un nombre d'insectes élevé sur les plantes). ii/ de rechercher les facteurs génétiques contrôlant cette résistance dans les populations de lignées d'introgression obtenues à partir de ces deux accessions par des tests de résistance au TYLCV après inoculation du virus par des aleurodes vérulifères en cages d'élevage de masse et/ou sous micro-cagettes. (Résumé d'auteur

Structuration génétique des populations et compétition larvaire au sein du complexe des Dacini

À La Réunion, les cultures de cucurbitacées subissent des dommages considérables en raison de l'attaque de mouche des fruits (Diptera : Tephritidae). Un complexe de trois espèces (Bactrocera cucurbitae, Dacus ciliatus et D. demmerezi) coexistant sur l'île est capable d'infester 16 espèces différentes de cucurbitacées. Une meilleure connaissance de l'écologie et de la génétique des populations est une condition préalable pour développer des méthodes de gestion rationnelles et durables contre ces ravageurs. Nos objectifs sont de (i) décrire la variabilité génétique présente chez B. cucurbitae et tenter de retracer ses processus de colonisation ; (ii) d'évaluer l'impact de la compétition larvaire pouvant exister au sein d'un même fruit hôte, tant au niveau intra- qu'interspécifique. Afin d'étudier la variabilité génétique et la structuration des populations de B. cucurbitae, des cucurbitacées infestées cultivés et sauvages ont été récoltées selon un gradient altitudinal pendant l'été et l'hiver 2009, puis ramenées au laboratoire et conservées jusqu'à l'émergence des adultes. Pour mesurer les effets de la compétition larvaire sur le taux de survie des larves et sur le poids des pupes, des infestations artificielles de courgettes ont été réalisées avec des larves de stade L1 selon cinq niveaux d'infestation. Nous avons montré l'existence d'une population bien différenciée de B. cucurbitae à La Réunion. Le continent africain semble être le point d'entrée majeur des invasions. D'autre part, nous avons mis en évidence l'existence de stratégies différentes des larves de ces trois espèces, face à une augmentation de la compétition au niveau intra- et interspécifique au sein d'un même fruit hôte. Alors que les larves de B. cucurbitae maintiennent une bonne survie, celles de D. demmerezi montrent une augmentation de la mortalité, mais le poids des pupes qui survivent demeure élevé. (Résumé d'auteur

Population genetics of two fruits flies damaging cucurbits on la Réunion :Bactrocera cucurbitae and Dacus ciliatus

#Bactrocera cucurbitae# and #Dacus ciliates# are two fruit fly species damaging cucurbit crops. Of Asiatic origin, #B. cucurbitae# has become an invasive pest in many countries especially on the African continent. Conversely, #D. ciliates#, of African origine, expanded its distribution area during the last years and is now becoming a pest in different Asiatic countries. Both species rank among invasive pests and have been introduced on La Réunion, a French island in the South-Western Indian Ocean, over the last 50 years. They now damage most cucurbits cultivated in the island and cause important yield losses. La Reunion is a subtropical island, with high altitudinal gradient (up to 3000 m) and two seasons (hot and rainy summer, milder and drier winter). Nevertheless, the two species are found in most areas, regardless of season and altitude. However, they show some altitudinal preferences: #B. cucurbitae# is more abundant in the lowlands and #D. ciliates# at medium altitudes. The aim of this work was to characterize the population structure of #B. cucurbitae# and #D. ciliates#, test host races and seasonal or altitude effects. (Résumé d'auteur

Population structure of the melon fly, Bactrocera cucurbitae, in Reunion Island

The melon fly, Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae) is an agricultural pest of major significance worldwide that primarily attacks cucurbit crops. In Reunion Island, it represents the main tephritid pest on cucurbits. In this paper, we provide a genetic characterization of populations of B. cucurbitae from Reunion Island and investigate their geographical origin using ten microsatellite loci at two mitochondrial gene fragments. Microsatellites reveal the occurrence of three different genetic clusters of B. cucurbitae in Reunion Island, all clearly distinguishable from their African and Asian relatives. These three clusters are sympatric and show no signs of recent bottlenecks. Levels of gene flow among clusters are relatively high, yet gene flow also occurs with populations from the African continent and, to a lesser extent, from Asia. The B. cucurbitae clusters show distinct distributions across eastern and western locations in Reunion Island (but not at different altitudes or between wild and cultivated host plants or between sampling periods), and their abundance is also correlated with the average amount of rainfall. Microsatellite and sequence analyses suggest Africa as the most probable source area for populations of B. cucurbitae in Reunion Island. (Résumé d'auteur